In the related art, as a steel sheet for an iron core of a transformer, there is known a grain-oriented electrical steel sheet that exhibits excellent magnetic characteristics in a specific direction. The grain-oriented electrical steel sheet is a steel sheet in which a crystal orientation is controlled so that a magnetization easy axis of a crystal grain and a rolling direction match each other by a combination of a cold rolling treatment and an annealing treatment. It is preferable that an iron loss of the grain-oriented electrical steel sheet is as small as possible.
The iron loss is classified into an eddy current loss and a hysteresis loss. In addition, the eddy current loss is classified into a classical eddy current loss and an anomalous eddy current loss. Typically, there is known a grain-oriented electrical steel sheet in which an insulating film is formed on a surface of a steel sheet (base metal) of which a crystal orientation is controlled as described above so as to reduce the classical eddy current loss. The insulating film also plays a role of applying electrical insulating properties, tensile strength, heat resistance, and the like to the steel sheet. Furthermore, recently, there is also known a grain-oriented electrical steel sheet in which a glass film is formed between the steel sheet and the insulating film.
On the other hand, as a method of reducing the anomalous eddy current loss, there is known a magnetic domain control method of narrowing a width of a 180° magnetic domain (performing refinement of the 180° magnetic domain) by forming a stress strain portion or a groove portion, which extends in a direction intersecting the rolling direction, at a predetermined interval along the rolling direction. In the method of forming the stress strain, a 180° magnetic domain refinement effect of a reflux magnetic domain, which occurs in the strain portion, is used. A representative method thereof is a method of using a shock wave or rapid heating with laser irradiation. In this method, a surface shape of an irradiated portion hardly varies. On the other hand, in the method of forming the groove, a demagnetizing field effect due to a magnetic pole, which occurs in a groove side wall, is used. In this case, it is known that when a cross-sectional shape of the groove is close to a rectangle, the magnetic domain control effect is high (Patent Document 4). That is, the magnetic domain control is classified into a strain applying type and a groove forming type.
In a case of manufacturing a wound core transformer by using the grain-oriented electrical steel sheet, it is necessary to perform a stress relief annealing treatment so as to remove a deformation strain which occurs when the grain-oriented electrical steel sheet is coiled in a coil shape. In a case of manufacturing a wound core by using the grain-oriented electrical steel sheet that is subjected to the magnetic domain control by the strain applying method, the strain is disappeared due to execution of the stress relief annealing treatment. Therefore, the magnetic domain refinement effect (that is, an anomalous eddy current loss reducing effect) is also lost.
On the other hand, in a case of manufacturing the wound core by using the grain-oriented electrical steel sheet that is subjected to the magnetic domain control by the groove forming method, the groove is not lost even when executing the stress relief annealing treatment. Accordingly, it is possible to maintain the magnetic domain refinement effect. Accordingly, in a method of manufacturing a magnetic domain control material for the wound core, the groove forming type is employed.
Furthermore, in a case of manufacturing a stacked core transformer, the stress relief annealing is not performed. Accordingly, it is possible to selectively employ any one of the strain applying type and the groove forming type.
Furthermore, in a laser method that is a representative method of the strain applying type, for example, as disclosed in Patent Document 5, when being irradiated with a laser having relatively high intensity, a steel sheet surface is slightly melted, and a moderate depression having a depth of approximately 10 μm may be formed. However, in the moderate depression as described above, there is no occurrence of a magnetic pole with which the magnetic domain control effect is obtained. As a result, it is known that the magnetic domain control effect is lost after the stress relief annealing.
As the groove forming type magnetic domain control method, typically, there are known an electrolytic etching method in which a groove is formed in a steel sheet surface of the grain-oriented electrical steel sheet through the electrolytic etching method (refer to Patent Document 1), a gear press method in which a groove is formed in a steel sheet surface by mechanically pressing a gear on the steel sheet surface of the grain-oriented electrical steel sheet (refer to the following Patent Document 2), and a laser irradiation method in which a groove is formed in a steel sheet surface of the grain-oriented electrical steel sheet through laser irradiation (refer to the following Patent Document 3).
In the electrolytic etching method, for example, an insulating film (or a glass film) on the steel sheet surface is removed in a linear shape with a laser or mechanical means, and then electrolytic etching is performed with respect to a portion at which the steel sheet is exposed, thereby forming a groove in the steel sheet surface. In a case of employing the electrolytic etching method, a process of manufacturing the grain-oriented electrical steel sheet becomes complicated. Therefore, there is a problem that the manufacturing cost increases. In addition, in the gear press method, since the steel sheet that is the grain-oriented electrical steel sheet is a very hard steel sheet containing 3 mass % of Si, abrasion and damage of the gear are likely to occur. In a case of employing the gear press method, when the gear is abraded, the groove becomes shallow, and a difference occurs in a groove depth. Therefore, there is a problem that it is difficult to sufficiently attain the anomalous eddy current loss reducing effect.
On the other hand, in a case of the laser irradiation method, direct working is performed. Accordingly, a complicated process such as etching is not necessary. In addition, non-contact type working is performed, and thus the same abrasion of the gear as in press working and the like do not occur. As a result, it is possible to stably form a groove on the steel sheet surface. For example, the following Patent Document 4 discloses a technology of improving the magnetic domain refinement effect (iron loss reducing effect) by making a groove contour shape (groove cross-sectional shape) on a cross section, which is perpendicular to a groove extension direction, close to a rectangle in the grain-oriented electrical steel sheet in which the groove is formed in the steel sheet surface according to a laser irradiation method.